Communication schemes have been well-known where clock signals (clocks), as well as data signals, are transmitted between transmitting and receiving devices.
One such example is the PCI Express® wherein clocks are transmitted in the side band upon transmitting bus signals in servers, personal computers (PCs), or devices. (As for the PCI Express®, refer to PCI express OCuLink Specification Rev 0.5).
The PCI Express® is one communication standard for performing communications via a pair of transmitting and receiving paths for serial transmissions (such paths are referred to as lanes).
The maximum speed of a single lane is 8 gigabytes (GB) per second (GB/s), and 32 lanes can be bundled at maximum. The bundle of the transmission paths are collectively referred to as a link.
A typical PCI Express® system includes a root complex, an end point, a switch, and a bridge.
The root complex is a device located at the bottom of the hierarchy, and includes a host bridge and is connected to a CPU, a memory, and the like.
In PCI Express®, an input/output (I/O) device is referred to as an end point.
In the meantime, the recent increase in data communication speeds has underlined the influences of the noises caused by electro magnetic interferences (EMIs).
For addressing such EMI noises, an increasing number of systems and/or architecture have adopted clocks having periodically varying frequencies, as standard clock, such as spread spectrum clocks (SSCs), as their standard clocks. SSCs are clocks where the bandwidth is spread by sweeping the clock frequency.
SSCs intentionally fluctuate the frequency of the clocks, thereby preventing energy to be concentrated on a particular frequency, which may cause EMIs to reduce noises.
FIG. 8 is a graph illustrating an SSC waveform used in a transmission system. This SSC represents a down spread wherein a triangular wave having a modulation frequency of 30 kHz, as specified in PCI Express®, is modulated only to the lower-frequency side, with a frequency of 100 MHz and a modulation index of 0.5%, for example.
As depicted in FIG. 8, since SSCs have frequencies that vary over time, the signal intensity for each frequency is reduced, which reduces the influences of EMIs.
In the meantime, demands for long-haul data transmissions have increased in recent years. For example, for interconnections between server racks, e.g., Internet data centers (iDCs) and centralized management of devices, inter-enclosure connections among peripheral devices, CPUs, and memory blades, located in separate enclosures, are increased. Furthermore, a desire to transmitting massive data to monitors at higher speed is increased in commercial PCs.
With a significant increase in the transmission distances, deviations between the changing frequency of SSCs and the frequency of received data have become problematic.
To address such deviations, techniques are proposed for minimizing the diffusion coefficient of SSCs. For example, there are techniques that minimize the diffusion coefficient of SSCs by checking the status of devices that are connected.
There is, however, a tradeoff between the diffusion coefficient and the transmission delay time. Hence, in systems where sufficient diffusion coefficients are required for SSCs, the SSCs cannot be transmitted in a longer distance.
FIG. 9 is a schematic diagram illustrating a configuration of a conventional data transmission system, and FIG. 9B a graph illustrating an SSC waveform used in the data transmission system 1 in FIG. 9A.
The data transmission system 1 uses signals having periodically changing frequencies (e.g., SSCs), as clocks.
The data transmission system 1 communicates data, in accordance with any suitable communication standard, such as the PCI Express®, for example. As depicted in FIG. 9A, the data transmission system 1 includes an upstream port 11, a downstream port 21, and an optical fiber 30.
The upstream port 11 may be a root complex of the PCI Express®, for example, which may be provided on a mother board of a server, for example. The upstream port 11 includes a reference oscillator 12.
The optical fiber 30 is a bundle of optical fiber cables, and includes a clock transmission path, a downstream data transmission path, and an upstream data transmission path.
The downstream port 21 may be an end point of the PCI Express®, for example, and may be provided in any suitable peripheral devices, such as hard disk drives, for example. The downstream port 21 includes a phase locked loop (PLL) 22.
Here, the clock is expressed in F(x) in FIG. 9B. The frequency of a clock, which is used for data generation by the upstream port 11 at Time t is expressed by F(t) and is indicated with the arrow in the double dashed line.
For t which is the time required for a signal to be transmitted from the upstream port 11 to the downstream port 21, the reference when the data is received in the upstream port 11 corresponds to the clock at the time when the data was generated in the downstream port 21, and the clock has been transmitted from the upstream port 11. In other words, the clock is the clock before 2T, and the frequency at that time is expressed in (t−2T) and is indicated with the arrow in the single dashed line FIG. 9A.
Therefore, in the upstream port 11, data generated using the clock with the 2T-old frequency F(t−2T) is strobed using the latest clock F(t).
In this manner, upon sending SSCs having a frequency which varies over time, occurrence of any transmission delay during transmission changes the frequency of the clock, thereby inducing the difference (deviation) in frequencies between the clock and a data signal. The greater the delay, the greater the frequency difference becomes, which is problematic for long-haul transmissions.
Specifically, if there is a difference between the reference clock of data and a clock used for strobing, the strobe point of the data may gradually deviate, which hinders the data from being received correctly.
For the above reason, in PCI Express® requires that the difference be within several hundred pars per million (ppm), for example.
In conventional configurations where hard disk drives are directly connected to a server, the difference in frequencies does not cause any problem since the transmission distance is small and hence T is negligible.
As set forth above, although demands for long-haul transmissions of SSCs become intense, an increased transmission delay hinders the SSCs from being transmitted for a longer distance.
In one aspect, the present disclosure is directed to enabling transmissions of timing signals having periodically varying frequencies in a longer distance.
Note that it is another object of the present embodiment to provide advantages and effects that can be obtained by the best modes to implement the embodiment described below but cannot be obtained with conventional techniques.